Horseshoe Crab-A living Fossil

Horseshoe Crab-A living Fossil

Limulus polyhemus

Because it has remained almost unchanged for millions of years, the Horseshoe Crab is often called a “Living Fossil”.  The Horseshoe Crab is one of the Earth’s oldest creatures, appearing over 100 million years before the dinosaurs! 

Long before the dinosaurs, there were hundreds of species of Horseshoe Crabs and their relatives, the sea scorpions.  The Horseshoe Crabs belongs to the group of animals called Arthropoda, which includes  lobsters, crabs, insects, spiders and scorpions.  Even though it looks crab-like, the Horseshoe Crab is more closely related to scorpions and spiders.  Today sea scorpions are extinct and only 4 species of Horseshoe Crabs remain. 

The Horseshoe Crab is also invaluable to human health. If you or someone you love has ever been hospitalized, it’s likely that the Horseshoe Crab played a role in your recovery.  A clotting factor in the crab’s blood, which can be removed without harming the crab, is used to detect bacteria in human blood, in intravenous drugs, and even in prosthetics such as heart valves prior to implantation. The crab’s blood is milky blue in color. The crab’s blood system can be drained, the crab is tagged then released back to sea. Amazingly, it will restore it’s blood supply  and return back to shore the following year.

1. The body is composed of three parts: the prosoma (head), opisthosoma (central area) and telson (tail).  The horseshoe crab’s name is derived from the prosoma, resembling the shape of a horse’s shoe.

2. The horseshoe crab has 10 eyes. There are two compound eyes visible on top of their shell that helps locate mates. In addition, each animal has eight other “eyes” located elsewhere on the prosoma, tail and underside of the body. These eyes sense visible and ultraviolet light, which are believed to allow the animal to respond to day-night and lunar cycles. 

3. The males can be distinguished from the females by the first pair of legs, which terminate in a hook-shaped structure used to grasp the shell of the female during mating. The females also are significantly larger than the male at maturity.

4. The horseshoe crab eats mollusks, worms, dead fish and algae. 

5. Horseshoe crabs are not crabs; they are more closely related to scorpions, ticks and land spiders. They are also related to trilobites that existed more than 500 million years ago.

6. Horseshoe crabs are believed to have inhabited the oceans for more than 350 million years. 

7. They have the ability to survive extreme temperatures and go for long periods without food, and their hard armor means that they do not have many predators other than sharks. 

8.  Horseshoe crabs play an important ecological role for many shorebirds that migrate through the U.S. Mid-Atlantic coast each spring on their annual migration from South America to their summer grounds in the Arctic. Eleven species of birds depend on horseshoe crab eggs as their primary food source during their two to three week stop-over. 

9. Horseshoe crabs have no jaws or teeth. Instead, they have an impressive array of spiny mouth bristles at the base of  five pairs of legs to maneuver food items such as razor clams, soft-shelled clams and marine worms into their centrally located mouth. To chew its food, the crab must simulate walking movements.

10. Horseshoe Crabs come to spawn on the high tide of the new and full moon in late spring and early summer. 

11. They require a sloping sandy beach to lay their eggs. 

12. It takes 9-12 years for a Horseshoe Crab to reach maturity, so all the Horseshoe Crabs you see on the beach are at least 9 years old and have a life span of 16-40 years. The Crabs grow by molting or shedding and emerge 25 % larger with each molt.  Molting occurs several time during the first two to three years and about once a year there after. After 16 molts (usually between 9 and 12 years) they will be fully grown adults. Females reach maturity 1 year later than males and consequently, go through an additional molt. Mature Crabs then repeat what has occurred for years, an annual spring migration to inshore spawning areas. They spend their life in the deeper water and when they reach maturity, the females move to the beaches only to spawn. 

13. The female, carrying the male, moves up on the beach, buries herself and lays 20,000-60,000 eggs, the male then fertilizes the eggs. Many of the eggs are eaten by birds and others animals and there is also a good chance of stranding on the beach. Single males that don’t find a female crowd around the mating pairs on the beach trying to fertilize some of the eggs. 

14. Adults males are two-thirds the size of the females and have a modified front pincher to attach to the female to mate.  The body is sometimes two feet long. Alive, it is brownish-green in color.  After it dies, the adult is dark brown. They breathe by means of gills attached to the underside of the last pair of abdominal legs.

15. The Horseshoe Crab can swim upside down in the ocean.

16. The Horseshoe Crab’s mouth, located on the under body and is surrounded by its legs and, while harmless, it is advisable to handle these Crabs with care or not at all.  The Crabs feed mostly at night and burrow for worms and mollusks. However, they will feed at any time. 

17. The Horseshoe Crab’s tail, while scary, is not a weapon. The tail is used to plow the crab through the sand and muck, to act as a rudder and to right the crab up when it accidentally tips over. If they cannot get turned back over, they will die.

18. Horseshoe Crabs have 2 compound eyes on the top of their shell with a sight range of about 3 feet. By recording electrical impulses from the crab’s optic nerve in its lateral eye, many principles underlying the functioning of all visual systems were discovered and gave Dr H. Keffer Hartline a shared part of the 1967 Nobel Prize. They have the ability to make their eyes one million times more sensitive at night and can see up to three feet away. 

20. Horseshoe Crabs also have a very important use for humans. The medical profession uses an extract from the Crabs blue, copper-based blood , called lysate, to test the purity of medicines. Certain properties of the shell have also been used to speed blood clotting and to make absorbable sutures.  Horseshoe Crabs have also to benefit cancer research. Pharmaceutical companies extract up to one-third of the animals blood volume and the animal is returned safely to the water and bled animals have a 90% survival rate. 

21. In addition, the chitin, or cellulose-like compound, in its shell is used in sutures and burn dressings to increase healing time.

22. Horseshoe crabs were dried for fertilizer and for poultry food supplements in the 1900s before artificial fertilizers were used.